1. Field of the Invention
This invention relates to a method of manufacturing a semiconductor device employed for reducing standing wave effects.
2. Description of Related Art
A stepper as a pattern projecting device used for photolithography employs a single wavelength KrF excimer laser with a wavelength of 248 nm as a light source and has a lens NA of approximately 0.37 to 0.42.
However, if exposure is carried out with the stepper, the single wavelength of the light source generates a phenomenon called standing wave effect. The generation of the standing wave is caused by light interference within a resist film. That is, the interference is caused within a film of a resist PR by an incident light P and a reflected light R from the boundary between the resist PR and a substrate S, as shown in FIG. 1. Consequently, the amount of light absorbed in the resist changes in accordance with the resist thickness, as shown in FIG. 2. The amount of light absorbed in the resist indicates the amount of light absorbed in the resist itself excluding the amount of a light reflected on the surface, a light absorbed in any metal or a light radiated from the resist. The standing wave effect depends upon a ratio of a change .DELTA.A in the amount of light absorbed in the resist to a change A in the resist thickness, as shown in FIG. 3. This .DELTA.A/A ratio is called a swing ratio.
In an actual device, pits and lands exist on the substrate surface. For instance, a recess composed of polysilicon or the like exists on the substrate surface, as shown in FIG. 4. For this reason, if the resist PR is applied thereon, the resist thickness differs between the upper side and the lower side of the step. In short, the resist thickness d.sub.PR2 above the recess is smaller than the resist thickness d.sub.PR1 of the other portion. Since the standing wave effect differs in accordance with the resist thickness, the amount of light absorbed in the resist affected by the standing wave effect also changes. Therefore, the size of a resist pattern produced after exposure and development differs above and below the step.
In order to reduce the above-described standing wave effect, use of antireflection layers 92 as shown in FIG. 5 has been considered to be effective. Conventionally, the antireflection layers 92 are formed below insulating layers 33 composed of offset oxidation films or the like.
A conventional semiconductor device to reduce the standing wave effect has first electrically conductive interconnection layers 91, and the antireflection layers 32, the insulating layers 33, interlayer insulating layers 34 and a second electrically conductive interconnection layer 35 which are formed on the first electrically conductive interconnection layers 31. The second electrically conductive interconnection layer 35 is bonded via a contact hole to the lower layer, that is, an N-type impurity diffused region 38 formed on the surface of a silicon substrate 37 in the example of FIG. 5.
Meanwhile, in the semiconductor device thus formed, the insulating layer 33 composed of the offset oxidation film or the like is diminished by etching and pre-processing for forming the contact hole 36. Therefore, it is necessary to deposit the insulating layer 33 to be relatively thick in consideration of irregularity in the etching. However, the thick insulating layer 33 generates a large step, and lithography and reactive ion etching (RIE) to the second electrically conductive interconnection layer 35 are difficult.
The antireflection layer 32 remains as it is. Therefore, if the antireflection layer 32 is formed by the low-temperature CVD method, such as Si.sub.x O.sub.y N.sub.z, containing hydrogen and water, the hydrogen and water are changed by heat treatment, deteriorating characteristics of devices such as a transistor.
If the insulating layer 33 is formed by the high-temperature CVD method, such as LP-TEOS, the antireflection layer 32 formed below the insulating layer 33 is optically transformed, disturbing satisfactory patterning by lithography.